EP0672581A1

EP0672581A1 - Tug boat for escort towing and/or harbour use

Info

Publication number: EP0672581A1
Application number: EP95850053A
Authority: EP
Inventors: Harri Kalevi Eronen
Original assignee: Aquamaster Rauma Oy
Current assignee: Kongsberg Maritime Finland Oy
Priority date: 1994-03-14
Filing date: 1995-03-13
Publication date: 1995-09-20
Also published as: KR950031774A; CN1115297A; CA2144460A1; FI941196A; US5575230A; FI941196A0

Abstract

The present invention relates to a tug boat intended for escort towing and/or harbour use, being provided with a towing winch (22) positioned at least on the forecastle. The tug (40) is intended, while in escort towing, to improve at high speed the steering and arresting properties of a vessel to be as-sisted by means of a tow rope (21) emitted from the towing winch (22) and connected to the vessel being assisted. In harbour use, the tug boat (40) can be applied for ordinary towing aand buffer tasks. In order to improve the stability of the tug boat (40) and the towing, steering, arresting and equivalent properties provided with the tug boat in the vessel being assisted, a towing eyelet (24) through which the traction force of the tow rope (21) connected from the towing winch (22) of the forecastle to the vessel being assisted is transmitted to the tug boat (40), has been arranged to be positioned, in accordance with the towing angle, to the proximity of the deck plane or in the deck plane. The hull of the tug boat (40) has moreover been so designed that the hydrodynamic point of application of the hull is brought up and to the front of the adjacency of the towing eyelet (24) to reduce the torque heeling the tug boat (40) and to bring the rope force and the hydrodynamic force longitudinally close to one another.

Description

The present invention relates to a tug boat intended for escort towing and/or for use in harbour, said tug boat being provided with at least a towing winch mounted on the fore-castle, said tug boat being intended, while escort towing, to assist at high speed the steering and arresting properties of a vessel to be assisted by means of a tow rope coming from the towing winch and connected to the vessel being assisted, and which tug boat can, while working in the harbour, be applied to normal towing and buffering tasks.
Accidents occurred in the immediate past, which may even have lead to major oil damages, have accumulated pressures towards improvements in safety in marine oil transports. Some of the accidents lead to oil damages were resulted in that an oil tanker lost either its steerability or propulsion thrust at a critical moment. As a consequence of such oil accidents, the requirements concerning tanker structures have been tightened, inter alia, so that a double bottom structure is required to be built in tankers. In addition, development of tug boats of a novel type has been necessary to enable assistance of tankers in dangerous and coastal waters.
Totally different standards are set for such, so-called escort tug boats compared with conventional harbour tug boats. First-ly, the escorting speed of an escort tug boat is requried to be at least as high as the lowest operating speed of a tanker. The most economical escorting speed is the highest permitted speed for tankers in a certain area, or, if no such limi-tations exist, the highest permitted speed at which the trafficing is safe. In practice, this means that the escorting speed can even be 13 to 14 knots. Accordingly, the tug boat is at this speed required to be able to carry out its escorting tasks in addition to that it merely follows the tanker at said speed. Furthermore, the escort tug shall have to be able to function in all weather conditions. Such prerequisites re-quire, firstly, that on an escort tug boat, action in all conceivable directions must be possible and, if needed, that it has to be able to change direction at maximum speed.
Furthermore, an escort tug boat like this is required to possess maximum traction power. Because of such requirements, the only useful propulsion apparatus in current escort tug boats is, in fact, a propeller means capable of turning around 360° and possessing a great propulsive thrust.
Primarily two types of tug boats appropriate for escort towing are known in the art, one of them being a so-called tractor tug boat in which the towing winch is positioned on the aft deck and in which the propeller means have been disposed on the front side of the towing winch, closer to the bow of the vessel. The other type is a so-called stern drive tug boat in which the towing winch is located on the fore deck and in which the propellers have been disposed in the stern of the vessel. The tractor tug boats and stern drive tug boats thus represent the state of art technology. A drawback particularly related to the stern drive tug boats is that although the lateral surface area of the hull thereof is rather large, it is unadvantageous in shape and the point of application of the force is located too far back so that no good transverse forces have been achieved.
In ordinary tug boats, mainly intended for towing only and not for arresting, an arcuate construction provided with a hook has generally been arranged on the aft deck of the tug boat to which hook the tow rope has been fastened. This has been found to increase the stability of the tug boat. On the forecastle of tug boats intended for arresting, no such constructions have been used.
On the other hand, a box keel or plate keel has frequently been used to improve the direction stability in ordinary vessels, but not in tug boats.
The object of the present invention is to provide a novel tug boat for escort towing and/or harbour use, whereby an improve-ment is achieved compared with the prior art designs. For im-plementing this aim, the invention is mainly characterized in that for improving the stability of the tug boat and the tow-ing, steering, arresting and equivalent properties to be pro-vided with a tug boat in a vessel to be assisted, a towing eyelet or equivalent is arranged, through which the traction power of the tow rope connected from the towing winch of the forecastle to the vessel to be assisted is transmitted to the tug boat, to be positioned, in accordance with the towing angle, to the proximity of the deck plane or in the deck plane, and that the hull of the tug boat has been so shaped that the hydrodynamic point of application of the hull can be provided up and before the adjacency of the towing eyelet or equivalent in order to reduce the torque heeling the tug boat and to bring the rope force and the hydrodynamic force in longitudinal direction close to one another.
With the invention, remarkable advantages are gained compared with designs known in the art. Of said advantages, a feature may, for instance, be disclosed that in the tug boat the traction point of a first traction rope of the winch wire has been arranged to be mobile so that said traction point is always at an optimal point regarding the stability of the tug boat. A second significant advantage lies therein that the side projection of the underwater part of the tug boat has been so designed and made so large that the tug boat is capable of receiving extremely powerful forces. Furthermore, the side projection of the underwater part of the vessel is such in shape that the pressure centrepoint of the projection can be arranged at an optimal point relative to the traction point of the winch. The other advantages and characteristic features of the invention become obvious below in the detailed description of the invention.
The invention is described below by way of an example, re-ferring to the figures of the accompanied drawing.

Fig. 1 presents schematically an elevational view of a tractor tug boat.
Fig. 2 presents schematically an elevational view of stern drive tug boat of the invention.
Fig. 2A presents schematically the bow part of the tug boat shown in Fig. 2.
Fig. 2B is a section of Fig. 2A along line B-B.
Fig. 3A,3B,3C and 3D present schematically various modes of operation of a tug boat.
Fig. 4 presents schematically a view of a tug boat in traction situation when viewed in the longitudinal axis direction of the tug boat.
Fig. 5 presents schematically in top view a traction arrangement of a tug boat of the invention.
Fig. 6 presents a schematical side view a part of a tug boat provided with an advantageous embodiment of the traction arrangement of the invention.
Fig. 7 is equivalent to Fig. 6 in top view.

In the schematical elevational view shown in Fig. 1, the tractor tug boat is in general indicated by reference numeral 1. As can be seen in Fig. 1, the propeller means 2 have been positioned closer to the bow than the aft in the tug boat, and however, on the front side of the traction point 5 of the tow-ing winch 4. The tow rope or wire is in Fig. 1 indicated by reference numeral 6. In the stern of the tug boat, a large stern fin 3 has been installed under the waterline W, the purpose thereof being to increase the side projection of the underwater hull profile of the tug boat so that the tug boat 1 is able to receive greater forces laterally. The purpose of the stern fin is also to improve the direction stability. In Fig. 1, the hydrodynamic point of application of the side projection is indicated by reference P. The location of said hydrodynamic point of application P is essential concerning the traction power of the tug boat 1 and the receptivity of such forces. As regards the traction power and the receptivity of the forces, the most important factors are the longitudinal and height-directional distance of the propeller means 2 from the traction point 5, as well as the longitudinal and height-directional distance of the hydrodynamic point of application P from the traction point 5. Said dimensions are of major significance considering traction power and the stability of the tug boat.
Fig. 2 presents as a schematical elevational view a stern drive tug boat, generally indicated by reference numeral 10. The stern drive tug boat 10 is provided with propellers pos-itioned in the stern of the tug boat while the towing winch 14 is positioned on the forecastle of the tug boat. The traction point is indicated by reference numeral 15 and the tow rope or wire by reference numeral 16. In a tug boat 10 as in Fig. 2, the transverse projection of the underwater hull profile of the tug boat has been produced large in that the tug boat 10 is provided with a bow bulging 12. Furthermore, an additional keel, e.g. box keel 13, plate keel or equivalent, has been mounted under the bottom of the vessel, which further increases the transverse projection of the hull profile. Thanks to the bow bulging 12, the hydrodynamic point of application P of the side profile can be moved forward to be closer to the traction point 15. Reference P' depicts where said hydro-dynamic point of application is located without a bow bulging 12. The surface of the water is indicated by reference W in Fig. 2.
It is noteworthy to point out at this point that the locations of the hydrodynamic points of application P,P' shown in Figs. 1 and 2 are not constant but that they shift, depending on the entry angle of the flow, longitudinally to the vessel. The hydrodynamic point of application P is typically located in a tractor tug boat 1, as shown in Fig. 1, in the range between the midway and stern of the vessel, and in a stern drive tug boat 10 as in Fig. 2, between the midway and bow point of the vessel. The points depicted in the figures are merely pre-sented by way of examples.
In conjunction with the description of Fig. 2 the effect of the bow bulging 12 and the additional keel 13 in enlarging the hull profile is introduced, and therethrough, increase of lateral traction powers. However, with the aid of the bow bulging 12 and the additional keel 13, it is particularly the "force" of the hull of the vessel which increases in substan-tially lateral traction situations, thus adding considerably to the force without an increase in the surface area of the side profile in the same proportion. The lateral force is typically doubled even by about 10% surface area addition with the aid of said additional projections, that is, the bow bulging 12 and the additional keel 13. This results in that a greater lateral force can be provided with a smaller and less expensive vessel/hull. Similarly, the point of application of the force, or the hydrodynamic point of application P can with the aid of said projections be kept as high as possible, whereby the heeling torque, described below e.g. in associ-ation with Fig. 4, and the draft remain smaller than by dis-posing a large-size fin of tractor tyg type under the vessel hull. The effect of shifting the hydrodynamic point of application P of the bow bulging 12 forward is preferred because the rope force and the hydrodynamic force are now brought closer to one another. The force reducing the rope force of the propellers can therethrough be minimized.
Fig. 2A shows schematically the bow part of a tug boat ac-cording to Fig. 2 in order to demonstrate the design of the bow bulging 12, and Fig. 2B shows a schematical sectioned view of Fig. 2A along line B-B, that is, at a point where the waterline shears the stem. As shown in Fig. 2A, the bow bulging 12 extends as far forward as possible. However, the bow bulging 12 has been so designed that in buffer situations, that is, in using the tug boat in harbour work it remains on the rear side of the bow of the vessel. The bow bulging 12 is preferredly flat, even plate-like, in order to increase the transverse force as effectively as possible. The lower edge of the bow bulging 12 is most preferredly sharp-angled, and simi-larly the front edge and the upper edge are relatively sharp so that the flow would disengage in inclined towing situations as well as possible, thus creating maximal transverse force. The bow bulging has, however, been formed preferably lens-like, as can be seen in Figs. 2A and 2B, so that in a normal forward driving situation it acts towards reducing the resist-ance and increasing the clear water speed. Similarly, the rounder shape of the bow bulging 12 makes the tug boat easier to manage when driving in the wake of the vessel being as-sisted.
The stem of a tug boat according to the present invention, particularly of an escort tug boat, is in steep angle to the waterline so that said angle β is, for instance, about 45° and furthermore, the bow of the vessel has been so formulated that the water ejection is large, in other words, the angle γ in Fig. 2B is great, e.g. of the order of magnitude 45° so that no water will reach the forecastle in the roll of the sea. The power of the bow bulging 12 can be added further by arranging a plate-like section 12A between the stem and the bow bulging 12. Since said additional part 12A is plate-like, it will not impair the seaworthiness, on the contrary, it increases the transverse power.
As is pointed out in the foregoing, an additional keel 13 is furthermore used under the bottom of the tug boat according to the invention. It is stated earlier that said additional keel 13 can be, for instance, a box keel, a plate or equivalent, but also a T-beam structure is appropriate for said purpose. The power effect of a T-beam-shaped or plate-like additional keel 13 is the same or even greater than with a box keel, but docking of the vessel may in such case turn out to be more problematic.
Figs. 3A to 3D present different modes of operation in which the tug boat 10 of the present invention is used in escort towing. Figs. 3A and 3B present the main modes of operation in which the propagation of a tanker T is arrested with a tug boat 10 and, if needed, it is stopped. Fig. 3A shows a situ-ation in which the propeller means 11 of the tug boat 10 have been so directed that the propulsive thrust provided therewith is in the direction of travelling. In said mode of operation, the tug boat 10 is kept in the same direction as the tow rope 16. The traction F is therefore created solely with the aid of the propellers 11. In this mode of operation, the traction power F is dependent on the speed of the tanker T. The greatest traction power obtained in the tests was about 1.5 to 1.6 times the static traction power of the tug boat. However, as mentioned above, this mode of operation cannot be used at very high speeds because when the traction power is provided solely with the aid of the propellers, along with sufficient increase in the speed of the tanker T the engine of the tug boat 10 will be overloaded excessively. If said excessive overloading occurs, the tug boat 10 must be turned from the position presented in Fig. 3A.
Fig. 3B shows another mode of operation in which the tug boat 10 is used also for direct arresting and holding of the tanker T. This mode of operation differs from the one shown in Fig. 3A in that the propeller means 11 have been turned 90° rela-tive to the travelling direction of the tug boat 10 so that the propeller means face each other. In this mode of oper-ation, when the engines are running idle, the arresting effect provided by the tug boat 10 is insignificant. But, when the engines of the tug boat 10 are run at full speed, the arrest-ing effect is, even at a very low speed (about 8 knots), equal to the highest static traction power obtainable with the tug boat 10. This has been proved in the tests accomplished. How-ever, when the speed increases, the arresting effect also in-creases substantially linearly. With this mode of operation, there is no similar risk of the engines being overloaded as in using the mode of operation shown in Fig. 3A; hence, the mode of operation introduced in Fig. 3B can be used effectively at high speeds. Another significant advantage achieved with this mode of operation is that hardly any side thrust component is created in the tug boat 10, whereby the reduction of speed will not cause any interference in steering the vessel being assisted, that is, the tanker T.
Fig. 3C presents a mode of operation in which the tug boat 10 has been turned mainly in transverse direction to the traction rope 16. This mode of operation is a so-called dynamic mode of operation, and therethrough an extremely good and powerful arresting and steering effect can be obtained, particularly if the side projection of the underwater hull profile of the tug boat is sufficient. The arresting effect is here provided par-ticularly with the aid of the hull of the tug boat 10. It is especially in this mode of operation in which the stability of the tug boat is of great importance, because if the location of the traction point of the tug boat 10 relative to the pressure centrepoint of the side projection of the underwater hull profile of the tug boat is poor, the tug boat may even capsize. As is mentioned in the foregoing, this mode of oper-ation can be used particularly when steering a tanker T with the equipment of its own is difficult or even impossible, whereby the tanker T can with the aid of a tug boat 10 be kept in desired direction.
Fig. 3D shows a mode of operation which is, in a way, a com-bination of the modes of operation of direct arresting and of dynamic steering. In this mode of operation both the hull of tug boat 10 and the propellers are used to assist in arrest-ing, and in addition, this mode of operation is used for steering the tanker T being assisted in the manner similar to the mode of operation shown in Fig. 3C. As regards safety, the mode of operation of Fig. 3D is preferred to the design in Fig. 3C because the stability of the tug boat 10 is superior in this mode of operation.
As becomes obvious in Figs. 3A to 3D, the tug boat is required to be able to provide traction force in a number of different directions in relation to the length of the tug boat 10. In addition, as it is described above, the stability of the tug boat 10 in certain situations, while in operation, is problem-atic when traction is directed at the tug boat 10 from a dif-ficult direction. Figs. 4 and 5 present a design by which the stability of the tug boat 10 is improved in difficult situations of operation. Fig. 4 presents a tug boat 10 viewed in longitudinal direction and Fig. 5 shows the tug boat 10 when viewed schematically in top view so that in each figure the traction is directed at the tug boat from the side.
As can be seen in these figures, the stability of the tug boat has been so improved that on a deck of a tug boat 10 (either on fore deck or aft deck, or even on both decks,) a tow arc 19 is mounted, said arc being comprised of a tubular or rail structure or equivalent. The tow arc 19 is most preferredly circular in shape, as is presented in Fig. 5. A sledge, slide or equivalent towing eyelet has been positioned on the tow arc 19 to be moving along the tow arc, through which eyelet a tow rope 16 has been arranged to pass so that said towing eyelet 15 creates a traction point from which the tow rope 16 passes to a vessel to be assisted. The tow rope 16 passes from the towing winch 14 into the towing eyelet 15 through a steering runner 20, most preferably located in the centrepoint of the tow arc 19 or substantially within the range of the centre-point. The structure is preferably such that the steering runner 20 has been formed in the vertical shaft 17 whereon a horizontal beam has been mounted and on the outer end of which horizontal beam 18 the towing eyelet 15 has been installed. This will stiffen and stabilize the structure even more. The tow arc 19 has been arranged most advantageously in the plane of the deck in that the towing loop 15 passes as close to the deck of the tug boat 10 as possible, the purpose thereof being to provide the traction point as low as possible.
The effect and advantage to be gained by means of the struc-ture shown in Figs. 4 and 5 becomes most obvious in Fig. 4. As depicted in Fig. 4, the tow rope 16 passes from the towing winch 14 to the towing eyelet 15 either direct or via the steering runner 20. The distance of the line of action of the traction force acting on the tow rope 16 from the hydrodynamic point of application P of the side projection of the under-water hull profile of the tug boat is indicated by reference d in Fig. 4. Reference d' refers to distance from the hydro-dynamic point of application P in an instance in which the traction point of the tow rope would be located in the steer-ing loop 20. Said distance d', producing thus a lever arm to the traction force acting on the tow rope, is considerably greater than distance d, whereby in said two instances the torque capsizing the tug boat 10 is considerably smaller when using the tow arc 19 of the invention than without any tow arc. If the tug boat 10 heeled further from what it is pre-sented in Fig. 4, the line of action of the traction force affecting the tow rope would move even closer to the hydro-dynamic point of application P or even to the opposite side thereof. In such case, the traction power would no longer possess the tendency to capsize the tug boat; instead, it would make attempts to straighten the tug boat. As was dis-closed above, the design shown in Figs. 4 and 5 is particular-ly advantageous, especially in inclined towing situations as shown in Figs. 3C and 3D.
Thus, Figs. 6 and 7 present an advantageous embodiment of the traction arrangement of the invention, as taught whereby the traction arrangement has been positioned on the forecastle of the tug boat 40. As in Figs. 6 and 7, a tow arc 23 has been disposed in the front part 30 of the forecastle, this being in its entirety reserved for the tow arc 23 so that no other con-structions are arranged within said area. The front part 30 of the forecastle is not provided with any reel, neither is the area intended to be moved upon. By said arrangement, the tow arc 23 can be arranged as low as possible. Said arrangement may also be applied on the aft deck of the tug boat in similar fashion.
The bulwark 26 of the vessel 40 terminates in the bow in the rear part of the tow arc 23, and it is drawn transversely in the form of transverse bulwark 27 across the forecastle to define the winch 22 and the rear part of the forecastle. The tow arc 23 has been preferredly arranged to be shifted hyd-raulically aside (not shown), so that passing the tow rope 21 through the eyelet 24 the tow arc 24 can be performing without having to cross the transverse bulwark 27 to the front part 30 of the forecastle. The side view shown in Fig. 6 demonstrates that the front part 30 of the forecastle rises towards the bow up so that a freeboard can be added on the bow of the vessel. This will not impair the heeling tendency of the vessel 40 because in inclined towing situation the tow rope 21 is di-rected to the side in the rear part of the tow arc 23 at point K which is located more below than the bow.
In the embodiment of the traction arrangement in which a hori-zontal beam or equivalent steering rod 25 is used in associ-ation with the tow arc 23, a roller arrangement or equivalent measurement tools (not shown) measuring the traction power of the tow rope 21 can readily be connected thereto. Placing said measurement tools on a free tow rope 21 is quite difficult to implement.
The invention is described above by way of an example, refer-ence being made to the figures of the accompanying drawing. The invention is not, however, limited to concern solely the examples presented in the figures, but various embodiments of the invention may vary within the scope of the inventive idea defined in the claims accompanied below.

Claims

A tug boat intended for escort towing and/or harbour use, said tug boat being provided at least with a towing winch (14,22) installed on the forecastle, said tug boat (10,40) is intended, while in escort towing at high speed, to improve the steering and arresting properties of a vessel (T) being as-sisted by means of a tow rope (16,21) from the towing winch (14,22) and connected to the vessel to be assisted (T), and said tug boat (10,40) can be applied while in harbour use to normal towing and buffering tasks, characterized in that for improving the stability of the tug boat (10,40) and the tow-ing, steering, arresting and equivalent properties obtained with the tug boat in the vessel (T) to be assisted, a towing eyelet (15,24), whereby the traction force of the tow rope (16,21) connected from the towing winch (14,22) to the vessel (T) being assisted is transmitted to the tug boat (10,40), has been arranged to be positioned, in accordance with the towing angle, to the proximity of the plane of the deck or in the plane of the deck, and that the hull of the tug boat (10,40) has been so designed that the hydrodynamic point of appli-cation (P) of the hull is brought up and before the adjacency of the towing eyelet (15,24) or equivalent, in order to reduce the torque heeling the tug boat (10,40) and to bring the rope force and the hydrodynamic force longitudinally close to one another.
Tug boat according to claim 1, characterized in that the towing eyelet (15,24) has been positioned to be moving along a tow arc (19,23) mounted on the forecastle of the tug boat (19,40).
Tug boat according to claim 2, characterized in that the tow arc (19,23) is a circular arc in shape in that in traction situations directed to the sides the towing eyelet (15,24) has been arranged to move from the centreline of the tug boat closer to the side of the tug boat.
Tug boat according to any one of the preceding claims, characterized in that the hull of the tug boat has been provided with a bow bulging (12) projecting forward from the stem.
Tug boat according to claim 4, characterized in that the forward dimension of the bow bulging (12) is maximized, though in that for making buffering situations possible, the bow bulging (12) is left on the aft side of the bow dimension of the tug boat (10,40).
Tug boat according to claim 4 or 5, characterized in that in order increase the transverse force, the bow bulging (12) is made flat.
Tug boat according to any one of claims 4 to 6, characterized in that the bow bulging (12) is lens-like in shape.
Tug boat according to any one of claims 4 to 6, characterized in that the bow bulging (12) is plate-like.
Tug boat according to any one of claims 4 to 6, characterized in that for improving the power of the bow bulging (12) and for increasing further the transverse force, a plate-like additional part (12A) has been installed between the bow bulging (12) and the stem of the tug boat (10,40).
Tug boat according to any one of the preveding claims, characterized in that an additional keel (13) has been mounted on the bottom of the tug boat (10,40).
Tug boat according to claim 2, characterized in that the additional keel is a box keel, plate keel, T-beam keel, or equivalent keel known in the art.